Here is the full list of all 50 Eldest Souls achievements.
Reach the ending without dying once.
Defeat Drakmur, the Nightmare while taking 0 damage
Defeat Aryana, First of the Moon while taking 0 damage.
Defeat Eos, God of Unity while taking 0 damage.
Gain the Kindled Heart of Fire
Defeat Hyem while taking 0 damage.
Defeat Azikel, God of Light while taking 0 damage.
Defeat Eksyll in Abyss Mode.
Defeat Drakmur in Abyss Mode.
Defeat Aryana in Abyss Mode.
Gain the Abyssal Eye
Defeat The Warden of Purity in Abyss Mode.
Defeat The Deer God in Abyss Mode.
Perform 3 Perfect Counters in a row against any Boss.
Defeat Hyem in Abyss Mode.
Have at least 10 Windleaves active at once.
Defeat Eos in Abyss Mode.
Defeat Azikel in Abyss Mode.
Defeat the Guardian in Abyss Mode.
Defeat the Watchdog while taking 0 damage.
Die to a rolling log.
Defeat Eksyll
Reach the Steps of Ascension
Defeat The Deer God and the Warden of Purity.
Defeat The Deer God and the Warden of Purity while taking 0 damage.
Gain the Blessed Trueice Prism
Obtain the Empty Vessel
Defeat Aryana, First of the Moon.
Hit enemies with a Shard attack 200 times
Defeat Drakmur, the Nightmare
Deliver the bag of crowns to Edelon.
Defeat Hyem of the Frigid Waste.
Hit enemies with a thrust attack 200 times
Gain the Blessed Feather of Dawn
Maintain 4 stacks of Berserker for 30s.
Reach Concord Sanctuary
Defeat Eos, God of Unity
Defeat Azikel, God of Light.
Retry a Boss 10 times in a row.
Hit an enemy with Windslide, Berserker Slash, or Counter 20 times.
Reach the Crossroads
Defeat the Guardian.
Defeat the Guardian while taking 0 damage.
Talk with Rass
Deliver the harp string to Edd
Defeat the Watchdog.
Die to a Boss.
Reach Crusader's Landing
Finish the game without activating any Shard or Infusions.
Gain the Blessed Hunter's Charm